Being thankful is strongly linked with both mental and physical health— and can help to relieve stress, depression and addictions, among other conditions.

But what is gratitude? Psychologists view it as being able to maintain a world view that appreciates the positive. That may sound like optimism, but unlike simply expecting the good, “appreciation” requires recognizing that happy outcomes are not just the result of your own hard work or moral uprightness, but depend on the efforts of others and, for the more spiritually-minded, on divine providence as well.

This makes it a fundamentally social emotion: you are grateful either to other people or to some sort of higher power with whom you can communicate. And if you do not behave graciously, ingratitude can cause relational problems, which could deny you the type of social support that is needed to protect against stress and depression.

Numerous studies now link counting one’s blessings to health. A recent analysis published in Personality and Individual Differences included nearly 1,000 Swiss adults, ranging from teenagers to people in their 80s. It found that physical health was strongly linked with gratitude, basically because it improved psychological health. Better psychological health meant that people were more likely to engage in health-promoting activities and to seek medical help when it was needed. Not surprisingly, this kept people in better mental and physical condition than if they engaged in self-destructive behaviors and avoided necessary medical care.

Among those who are more spiritual, religious thankfulness, or gratitude toward God, can predict susceptibility to mental illness. In a 2003 study involving 2600 adults, those who were most spiritually thankful had a lower risk of depression, generalized anxiety disorder, phobias, bulimia and addictions including alcohol, nicotine and illegal drugs.

Of course, it’s possible that mentally healthier people feel that they have more to be grateful for, which may explain some of their extra thankfulness. However, because interventions aimed at improving gratitude seem to help with many of health conditions, it’s clear that whatever the reason, being thankful seems to have a strong relationship with health. Studies show, for example, that interventions to increase gratitude improve impaired body image by 76% and can help treat generalized anxiety disorder in similarly dramatic fashion.

So how does gratitude improve health? On one level, it helps people to sleep better. Since disturbed sleep is linked to almost all mental illnesses, factors that improve sleep tend to alleviate some of these disorders. A 2009 study of 401 people— 40% of whom had clinical sleep disorders— found that the most grateful people had better sleep quality, normalized sleep duration (not too long or too short), were able to fall asleep faster at night and also had less daytime tiredness compared to those who weren’t as thankful.

The key to reaping gratitude’s benefits seemed to involve what people thought about as they tried to fall asleep: while grateful folks accentuated the positive, the others were consumed by worries and fears. So mentally counting blessings before drifting off can help fight anxiety and depression, not just by replacing depressive and anxious thoughts but by making refreshing sleep easier to attain.

The most common ways to improve gratitude— making “gratitude lists” or keeping a daily diary focused on the things you are grateful for — build on this positive-focused thinking and are often a critical part of 12-step programs for addictions.

And they are effective, as a study tracking feelings of thanks and school satisfaction among a group of sixth and seventh graders showed. In the study, 221 children were assigned to write either a daily list of five things they were most grateful for, or of the hassles they experienced, or no list at all. The gratitude group reported greater satisfaction with school three weeks later compared to the other kids, especially those who focused on hassles. That’s a potentially significant benefit since contentment at school is linked to academic performance and dissatisfaction is correlated with antisocial behavior like drinking and drug use. The authors write, “[T]hese findings suggest that gratitude has both immediate and long-term effects on positive psychological functioning.”

And those effects may be self-sustaining to a certain extent as well. One study found that compared to those who didn’t experience extensive thankfulness, grateful people saw the help they received from others as being more costly to the giver and more valuable to themselves. In addition, they also interpreted deeper expressions of kindness and caring from these acts. These perceptions are likely to make people behave more gratefully towards others — since if you perceive the help you receive as being of little worth and primarily the result of self interest, you are less likely to be appreciative.

That may explain why gratefulness is a desirable trait in friends and colleagues, and why attempts to become more grateful can be an important part of improving many relationships.

So while it’s easy to focus on grievances during the hectic holiday season, try introducing a little gratefulness instead. Sure, there may be a bit of selfishness in that, since you may be motivated primarily to improve your own health, but it turns out that gratitude can change your perspective — in a contagious way that may ultimately help more than just you alone.